Clusters of Transmembrane Residues Are Critical for Human Prostacyclin Receptor Activation

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Abstract
Relaxation of vascular smooth muscle and prevention of blood coagulation are mediated by ligand-induced activation of the human prostacyclin (hIP) receptor, a seven-transmembrane-domain G-protein-coupled receptor (GPCR). In this study, we elucidate the molecular requirements for receptor activation within the region of the ligand-binding pocket, identifying transmembrane residues affecting potency. Eleven of 30 mutated residues in the region of the ligand-binding domain exhibited defective activation (decreased potency). These critical residues localized to four distinct clusters (analysis via a rhodopsin-based human prostacyclin receptor homology model). Residues Y752.65 (TMII), F953.28 (TMIII), and R2797.40 (TMVII) comprised the immediate binding-pocket cluster and were shown to be essential for proper receptor activation, compared to equivalent expression levels of the wild-type hIP (WT EC50 = 1.2 ± 0.1 nM; Y752.65A EC50 = 347.3 ± 62.8 nM, p < 0.001; F953.28A EC50 = 8.0 ± 0.6 nM, p < 0.001; R2797.40A EC50 = 130 ± 63.0 nM, p < 0.001). Residues S201.39 (TMI), F241.43 (TMI), and F722.62 (TMII) were localized to a cluster involving P171.36, a critical residue thought to facilitate transmembrane movement during changes in activation conformation. A third cluster formed around amino acid D602.50 (TMII), containing the highly conserved (100% of prostanoid receptors) D2887.49/P2897.50 motif located in TMVII. Last, a large hydrophobic cluster composed of aromatic residues F1464.52 (TMIV), F1504.56 (TMIV), F1845.40 (TMV), and Y1885.44 (TMV) was observed away from the ligand-binding pocket, but still necessary for hIP activation. These results assist in delineating the potential molecular requirements for agonist-induced signaling through the transmembrane domain. Such observations may be generally applicable, as many of these clusters are highly conserved among the prostanoid receptors as well as other class A GPCRs.